In the production of semiconductor LSI, a phase-shift mask is used as one of photo masks for the transfer of fine patterns. A half-toned phase-shift mask which comes under the above phase-shift mask is known as being suitable for the transfer of a single hole or dot or an isolated pattern such as a line or space.
In the above half-toned phase-shift mask, a mask pattern formed on the surface of a transparent substrate is composed of a light-transmitting portion for transmitting light having an intensity which contributes to substantial exposure and a light-semi-transmitting portion for transmitting light having an intensity which does not contribute to the substantial exposure, and the phase of light which passes through the light-transmitting portion and the phase of light which has passes through the light-semi-transmitting portion are differentiated, whereby light beams which have been transmitted offset each other in the vicinity of a boundary between the light-transmitting portion and the light-semi-transmitting portion so as to keep up a good contrast in the boundary. For example, JP-A-5-127361 discloses a half-toned phase-shift mask which gives a phase difference of 180.degree..
In the half-toned phase-shift mask disclosed in the above Laid-open Publication, the light-semi-transmitting film constituting the light semi-transmitting portion is composed of a monolayer film formed of a material having a homogeneous composition, such as a film of CrO.sub.x, CrN.sub.x, CrO.sub.x N.sub.y, CrO.sub.x N.sub.y C.sub.z, or the like.
When compared with a half-toned phase-shift mask of which the light semi-transmitting portion has a multi-layered laminate structure formed of a layer having a high transmittance (e.g., spin on glass called SOG for short) and a layer having a low transmittance (e.g., chromium), the above half-toned phase-shift mask having a light semi-transmitting portion formed of a monolayer film has advantages in that the production steps thereof are decreased in number and simplified, that the vertical construction in a pattern form is attained and that the imperfection generation rate can be decreased.
Concerning the light semi-transmitting film of CrO.sub.x, CrN.sub.x, CrO.sub.x N.sub.y, CrO.sub.x N.sub.y C.sub.z, or the like, the above Laid-open Publication discloses a method in which chromium is used as a sputtering target and an oxide, a nitride, an oxide/nitride or an oxide/nitride/carbide of chromium is deposited on a transparent substrate in the presence of a gas of oxygen, nitrogen, etc., in a deposition atmosphere; i.e., a so-called reactive sputtering method.
The light semi-transmitting film of CrO.sub.x, CrN.sub.x, CrO.sub.x N.sub.y, CrO.sub.x N.sub.y C.sub.z, which is formed by the above sputtering method, is poor in electrical conductivity when it is so formed as to simultaneously satisfies the property of transmitting light to an extent that the light does not contribute to exposure, i.e., the requirement that the transmittance to exposure light should be 4 to 20% and the requirement that the light semi-transmitting portion should give a predetermined phase difference. The light semi-transmitting film therefore has a problem that, in writing on a resist with an electron beam for patterning the resist provided thereon, shot electrons are charged (charge-up) in the resist so that no accurate pattern can be formed.
Further, the poor electrical conductivity causes an electrostatic charge, and there is a problem that a pattern is broken when the mask is produced or used, or that foreign matter or dust is likely to be adsorbed. The defect is therefore that, for preventing the charging phenomenon, therefore, it is required to form an electrically conductive layer which conducts and diffuses electricity, for example, on the transparent substrate, which increases the production steps in number.
Furthermore, there is another problem below. The electrically conductive layer formed between the transparent substrate and the light semi-transmitting film is required be transparent to exposure light. In particular, for coping with exposure light having a shorter wavelength, which is used as patterns having a high resolution are demanded in recent years, it is required to develop a new electrically conductive material transparent to the above exposure light.
The present inventors have already proposed a light semi-transmitting film containing a transition metal such as tungsten, tantalum or chromium and silicon as a substitute for the above chromium-containing light semi-transmitting film which is poor in electrical conductivity. The present inventors have been therefore able to provide a phase-shift mask blank which has both a light semi-transmitting characteristic and electrical conductivity by the formation of a light semi-transmitting film containing a transition metal and silicon on a transparent substrate and which overcomes the above problem of charging of a resist in patterning by writing with an electron beam, the breakage of a pattern at the mask production step or at the time of use and the problem of foreign matter or dust adsorption.
Meanwhile, when the above light semi-transmitting film containing a transition metal and silicon is formed on a transparent substrate by a sputtering method, generally, an oxygen gas is sometimes introduced together with an inert gas such as an argon gas. The purpose in introducing an oxygen gas together with an inert gas is to adjust the properties of the light semi-transmitting film such as transmittance, refractive index and electrical conductivity (resistance) to desired values by adjusting the oxygen gas amount ratio to control the amount of oxygen to be taken into the film. However, the introduction of an oxygen gas in sputtering newly causes the following problem. That is, a phenomenon takes place, in which the oxygen gas introduced at a spattering time oxidizes a spattering target surface, where an oxide grows, and the oxides bursts open due to an electric field charged for the sputtering and splashes on the substrate surface in a large cluster. This phenomenon is what is called "cluster adhering by abnormal discharging". The cluster not only causes dust and particles, but also forms convex portions. On the other hand, when the film formation proceeds in a state in which the cluster adhering to the substrate is included, and when the cluster then peels off at this stage, convex portions are formed. And, the concavo-convex form formed on the surface of the light semi-transmitting film invites the following defects.
1 A spatial invariability in phase shift amount is caused.
2 The transmittance of the light semi-transmitting film is decreased due to the irregular reflection of exposure light.
3 When the blank surface contacts other substance, at a handling time in particular, there is generated dust which causes a decrease in exposure performance.
4 The formation of a finer pattern is hampered.
It is therefore an object of the present invention to provide a phase-shift mask blank which overcomes the above problems 1 to 4 and a process for the production thereof.
It is another object of the present invention to provide a phase-shift mask obtained by patterning the light semi-transmitting film of the above phase-shift mask blank.